I personally love all the supplements form Excel Supplements. www.excelsupplements.com
Firstly deworm and check your horses teeth. Next check your hay quality and increase amount. Lastly I like a muscle bulder like MyoPower or MyoPlast. Both available from your vet. The larger tub of MyoPower is the cheapest way to go and works really well to put some weight and muscle on.
I have started my horse on a supplement called NRF2 and it is amazing. It is a herbal antioxidant that is regulating oxidative stress and aiding in weight gain. I can tell you more about it if you would like to know.
Copra Coolstance - although I’m Australian I am almost positive you guys can get Copra.